Nickel silicon magnesium alloy

ABSTRACT

A nickel-base alloy, useful as a negative thermoelement, is disclosed. The alloy consists essentially of from about 3.25 to about 5 percent by weight of silicon and from about 0.03 to about 0.25 percent by weight of magnesium and the balance nickel. The new alloy is more oxidation resistant and has a greater EMF stability than existing type K negative thermoelements.

United States Patent [191 Starr et al.

[111 3,820,983 June 28, 1974 NICKEL-SILICON MAGNESIUM ALLOY [73] Assignee: Wilbur B. Driver C0., Newark, NJ.

22 Filed: Aug. 20, 1971 [21 Appl. No; 173,671

52 U.S.Cl. ..75/170 51 Int. Cl. ..C22c 19/00 58 FieldofSearch 75/170, l7l;148/32, 32.5

[56] References Cited UNITED STATES PATENTS 1,076,438 l0/l9l3 Marsh ..75/l70 6/1935 Hunter 75/170 2/1968 Turillon et al. 75/170 Primary Examiner-Richard 0. Dean Attorney, Agent, or Firm-Norman J. OMalley; Donald R. Castle; William H. McNeil] [5 7] ABSTRACT A nickel-base alloy, useful as a negative thermoelement, is disclosed. The alloy consists essentially of from about 3.25 to about 5 percent by weight of silicon and from about 0.03 to about 0.25 percent by weight of magnesium and the balance nickel. The new alloy is more oxidation resistant and has a greater EMF stability than existing type K negative thermoelements.

2 Claims, No Drawings BACKGROUND OF THE INVENTION This invention relates to nickel-base alloys. More particularly, it is related to nickel-base alloy having a superior oxidation resistance and EMF stability under long time periods of usage than existing type K negative thermoelements.

Nickel-base alloys have been in use as thermoelements since the early part of this century. One of the commonly used thermocouples is the type K thermocouple (designated by the Instrument Society of America). This positive type K thermoelement is a nickel base alloy containing 9.25 Cr, 0.4 Si, balance essentially nickel. The negative type K thermoelement is a nickel base alloy containing 2A], 2.5 Mn, 1 Si, with small amounts of Fe and Co. There are numerous commercial suppliers of type K thermocouples.

The type K thermocouple is recommended to be used in air atmosphere. At high temperatures in air the nega tive type K thermoelement is the weaker link of the couple. lt.fails much sooner than the positive type K thermoelement because of its poorer oxidationresistance. In addition, a much larger portion of the EMF drift of the couple as a whole is attributed to the EMF change of the negative thermoelement. As a result, there is a need for a new negative thermoelement with improvedoxidation resistance and EMF stability.

It is believed, therefore, that a new alloy useful as a negative thermoelement which has excellent oxidation resistance and :EMF stability is an advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION from about 3.25 percent to about 5.0 percent by weight of silicon, from about 0.03 percent to about0.2 percent by weight of magnesium and the balance nickel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention together with other and further objects, advantages,-

and capabilities. thereof, reference is made to the following specification and appended claims.

The alloys of this invention have excellent oxidation resistance. Although the EMF of the alloys are more negative than .theprior art negative typeK thermoelements and when used with conventional positive type Kthermoelements. the resulting couple does not match the reference tables now in use, the trend is away from manual matching because the installation of many computer controlled processes. Thus, A EMF stability of the thermocouple has become more important than the actual A EMF at a particular temperature. It has been found that the alloys of this invention change very little even after being exposed in air from over 21,000 hours at l.800 F. Whencompared to the negative thermoelement now in use, the error in a thermocouple is greatly reduced.

Addition of silicon to nickel is known to improve oxidation resistance over the thermoelements now in use,

hence longer life; however, the alloys of this invention containing magnesium exhibit better EMF stability than alloys without the magnesium.

The alloys of this invention consist essentially of from about 3.5 to about 5 percent by weight of silicon and from about 0.03 percentto about 0.25 percent by weight of magnesium and the balance nickel. Preferred are alloys containing from about: 3.75 to about 4.25 percent by weight of silicon and from about 0.10 to about 0.20 magnesium with balance of nickel. Especially preferred is an alloy containing 4 percent silicon, 0.15 percent magnesium and balance nickel.

EXAMPLE I EMF at [800 F. vs. Pt MV A EMF A 2 Individual Thcrmoelement Initial After 2I'.l96 Hrs. in MV K +32.098 +3l.974 +.l24 K -'8.643 t '91168 .425 Ni/4Si' I().2U8 l0.487 -17) Ni/4Si/.I5Mg I().()92 -I0,l35 .043

*positive type K thermoelement negative type K thermoelement 1B! Thermocouple After 21.196 Hrs. A Temp.

A EMF in MV in I K ('I') VS. K +54) +25 2 K vs. Ni/4Si +2403 +18 3 K vs. Ni/4Si/.l5Mg +.I67 +7 of this invention by an alloy consisting essentially of l l Fora type K couple (1) the thermoelectric power at 1,800 F is 0.022 MV per F and the other two thermocouples 2 and 3) the thermoelectric power is 0.023 MV per F.

The above long term test at a high temperature of l,800 F decisively demonstrates that the Ni/4Si/0.l5 Mg alloy is far superior to the others to both the conventional type K thermoelement and to a 4 percent silicon-nickel alloy.

Thebeneficial effect in EMF stability is attributed to the addition of the proper amount of magnesium to the Ni/4Si alloy base.

The range of magnesium content is from 0.03 to about 0.25% Mg, with the preferred amount of from 0.10 to 0.20% Mg. The presence of Mg in nickel in trace amounts of 0.01 percent or below does not have percent to about 4.25 percent by weight of silicon and from about 0.1 percent to about 0.20 percent by weight of magnesium and balance nickel.

2. An alloy according to claim 1 wherein said silicon is about 4 percent by weight and said magnesium is about 0.15 percent by weight. 

2. An alloy according to claim 1 wherein said silicon is about 4 percent by weight and said magnesium is about 0.15 percent by weight. 